1. Field of the Invention
This invention relates to an electrophoretic display device. More specifically, the invention relates to an electrophoretic display device which forms an image through the electrophoretic controlled phase separation of a polar phase and a non-polar phase forming an emulsion.
2. Description of Related Art
Cathode ray tube (CRT) displays are frequently used in computer applications because of their speed and ease of scanning. Cathode ray tubes, however, are bulky and thus are not ideal for use in portable devices such as portable computers. CRT displays also have relatively high energy requirements which make these displays impractical for use in portable devices such as portable computers, personal portable information systems, modems, pagers, mobile phones, television monitors and other hand held devices. A further problem associated with using CRT displays is the difficulty associated with designing a CRT having a flat display panel.
Liquid crystal displays (LCD) have the advantages over CRT displays of providing a flat display panel which can produce images using relatively little energy. Liquid crystal displays, however, have the disadvantage that they generally provide a narrow field of view, enabling the user to view the display over a relatively narrow range. Liquid Crystal displays providing a wider field of view have been designed. However, these displays generally require polarizers which make the displays more expensive and complicated to manufacture. LCDs like Guest-Host, Nematic-Cholesteric Phase Change and light scattering by micron-size droplets do not require polarizers. However, these displays are slow and sensitive to temperature changes.
Electrophoretic image displays have been developed as an alternative to CRT and LCD displays, particularly for portable display applications. In general, electrophoretic image displays provide the advantages of requiring significantly less power than CRT displays and can be viewed over a wider field of view than LCD displays.
In the past, electrophoretic image displays have relied on the separation of charged particles dispersed in a dispersion medium where the particles are moved into and out of a dispersion using an electric field. The particles serve to either contribute a color or the absence of a color to the display.
Pixels are defined in an electrophoretic display as a region of the display in which the movement of particles, or other material forming a color (or absence of a color) of the display, is independently controlled relative to other regions of the display. Pixels are most commonly formed by a series of pairs of electrodes, each pair of electrodes forming a separate pixel. When a voltage potential is applied across a specific pair of electrodes, the electric potential draws the charged particles to a particular electrode. When no voltage is applied, the charged particles remain dispersed in the dispersion fluid, the color of the pixel being controlled by whether the charged particles are dispersed.
The charged particles used in electrophoretic image displays have a tendency to aggregate and/or adhere to the electrodes, reducing the useful life of the display as well as reducing the contrast ratio that can be achieved. The formation of stable dispersions using particles can also be difficult. For example, it can be difficult to match the specific densities of the liquid and the particles to form a stable dispersion. In addition, the image response rate achieved by electrophoretic displays using charged particles is limited by the rate at which the particles can be drawn into and out of the dispersion fluid.
The achievement of multi-colored electrophoretic displays using particles is limited since the particles do not generally allow light to pass through the particles. As a result, it is generally not practical to superimpose a series of cells in which particles are used as pigments.
A need currently exists for an electrophoretic image display system in which the image is formed without the use of particles. A need also exists for an electrophoretic image display which employs pigments which do not aggregate or adhere to the electrodes. A need also exists for a display in which the pigments are rapidly and stably dispersed in a dispersion medium. A further need exists for a display which employs a pigment--dispersion system having an improved response rate over electrophoretic displays using particles. Yet a further need exists for a multicolor electrophoretic display in which the pigments do not scatter light so that multiple pixels of different colors can be superimposed.